import java.util.ArrayList;
import java.util.List;

/*
 * @lc app=leetcode.cn id=17 lang=java
 *
 * [17] 电话号码的字母组合
 *
 * https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/description/
 *
 * algorithms
 * Medium (53.01%)
 * Likes:    632
 * Dislikes: 0
 * Total Accepted:    93.9K
 * Total Submissions: 177.1K
 * Testcase Example:  '"23"'
 *
 * 给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。
 * 
 * 给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
 * 
 * 
 * 
 * 示例:
 * 
 * 输入："23"
 * 输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
 * 
 * 
 * 说明:
 * 尽管上面的答案是按字典序排列的，但是你可以任意选择答案输出的顺序。
 * 
 */

// @lc code=start
class Solution {

    String[][] arr = new String[][]{
            {}, {}, {"a", "b", "c"}, {"d", "e", "f"}, {"g", "h", "i"}, {"j", "k", "l"}, {"m", "n", "o"}, {"p", "q", "r", "s"}, {"t", "u", "v"}, {"w", "x", "y", "z"}
    };

    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<>();
        if (digits.length()==0 || digits.contains("1")) {
            return res;
        }
        dfs(digits, 0, new StringBuilder(), res);
        return res;
    }

    void dfs(String t, int index, StringBuilder s, List<String> r) {
        if (s.length() == t.length() || index == t.length()) {
            r.add(s.toString());
            return;
        }

        char p = t.charAt(index);
        String[] temp = arr[p - '0'];

        for (int i = 0; i < temp.length; i++) {
            s.append(temp[i]);
            dfs(t, index+1, s, r);
            s.deleteCharAt(s.length()-1);
        }
    }
}
// @lc code=end

